Basic Principles of Batteries and Electrochemical Cells
Batteries may be divided into two principal types, primary batteries and secondary batteries. Primary batteries may be used once and are then exhausted. Secondary batteries are also often called rechargeable batteries because after use they may be connected to an electricity supply, such as a wall socket, and recharged and used again. In secondary batteries, each charge/discharge process is called a cycle. Secondary batteries eventually reach an end of their usable life, but typically only after many charge/discharge cycles.
Secondary batteries are made up of an electrochemical cell and optionally other materials, such as a casing to protect the cell and wires or other connectors to allow the battery to interface with the outside world. An electrochemical cell includes two electrodes, the positive electrode or cathode and the negative electrode or anode, an insulator separating the electrodes so the battery does not short out, and an electrolyte that chemically connects the electrodes.
In operation the secondary battery exchanges chemical energy and electrical energy. During discharge of the battery, electrons, which have a negative charge, leave the anode and travel through outside electrical conductors, such as wires in a cell phone or computer, to the cathode. In the process of traveling through these outside electrical conductors, the electrons generate an electrical current, which provides electrical energy.
At the same time, in order to keep the electrical charge of the anode and cathode neutral, an ion having a positive charge leaves the anode and enters the electrolyte and a positive ion also leaves the electrolyte and enters the cathode. In order for this ion movement to work, typically the same type of ion leaves the anode and joins the cathode. Additionally, the electrolyte typically also contains this same type of ion. In order to recharge the battery, the same process happens in reverse. By supplying energy to the cell, electrons are induced to leave the cathode and join the anode. At the same time a positive ion, such as Li+, leaves the cathode and enters the electrolyte and a Li+ leaves the electrolyte and joins the anode to keep the overall electrode charge neutral.
In addition to containing an active material that exchanges electrons and ions, anodes and cathodes often contain other materials, such as a metal backing to which a slurry is applied and dried. The slurry often contains the active material as well as a binder to help it adhere to the backing and conductive materials, such as a carbon particles. Once the slurry dries it forms a coating on the metal backing.
Unless additional materials are specified, batteries as described herein include systems that are merely be electrochemical cells as well as more complex systems.
Several important criteria for rechargeable batteries include energy density, power density, rate capability, cycle life, cost, and safety. The current lithium-ion battery technology based on insertion compound cathodes and anodes is limited in energy density. This technology also suffers from safety concerns arising from the chemical instability of oxide cathodes under conditions of overcharge and frequently requires the use of expensive transition metals. Accordingly, there is immense interest to develop alternate cathode materials for lithium-ion batteries. Sulfur has been considered as one such alternative cathode material.